This invention generally relates to micro-electro-mechanical systems (MEMS) and, in particular, to an improved electrothermal actuator for a micro-electro-mechanical device.
Electrothermal actuators are used in micro-electro-mechanical devices to provide force to move elements of the micro-electro-mechanical device. Electrothermal actuators use ohmic heating (also referred to as Joule heating) to generate thermal expansion and movement. Electrothermal actuators are typically capable of providing lateral deflections of eight microns (8 xcexcm) to ten microns (10 xcexcm). A micron is one millionth of a meter. Electrothermal actuators typically require drive voltages of approximately five volts (5 v).
FIG. 1 illustrates a perspective view of a prior art thermal beam actuator 100 mounted on a dielectric substrate 110. Micro-electro-mechanical systems (MEMS) technology is used to form thermal beam actuator 100 from a layer of polysilicon deposited on a dielectric substrate 110 such as silicon nitride. The components of thermal beam actuator 100 are formed from a common layer of polysilicon.
Thermal beam actuator 100 comprises first arm 120 and second arm 130. First arm 120 and second arm 130 are joined together at one end with a rigid polysilicon mechanical link 140. The end of thermal beam actuator 100 that comprises mechanical link 140 is able to move laterally and parallel to the surface of substrate 110. This end of thermal beam actuator 100 is therefore referred to as the xe2x80x9cfreexe2x80x9d end.
The other end of first arm 120 is coupled to anchor 150 and the other end of second arm 130 is coupled to anchor 160. Anchor 150 and anchor 160 are in turn coupled to substrate 110. This end of thermal beam actuator 100 is therefore referred to as the xe2x80x9cfixedxe2x80x9d end.
As shown in FIG. 1, thermal beam actuator 100 is formed having portions that define a gap 170 between first arm 120 and second arm 130. Gap 170 is formed by an interior edge of first arm 120 and by an interior edge of second arm 130. The width of gap 170 is determined by the width of mechanical link 140. Air in gap 170 provides electrical insulation between first arm 120 and second arm 130.
The width of second arm 130 is greater than the width of first arm 120 for most of the length of thermal beam actuator 100. As shown in FIG. 1, thermal beam actuator 100 is formed having portions that define a flexure portion 180 of second arm 130. Flexure portion 180 usually has a width that is the same width as first arm 120. A first end of flexure portion 180 is attached to anchor 160 and a second end of flexure portion 180 is attached to the end of the wide portion of second arm 130 that is adjacent to flexure portion 180.
Electric current (from an electrical source not shown in FIG. 1) may be passed through anchor 150, through first arm 120, through mechanical link 140, through second arm 130, through flexure portion 180, through anchor 160, and back to the electrical source. Alternatively, electric current (from an electrical source not shown in FIG. 1) may be passed through anchor 160, through flexure portion 180, through second arm 130, through mechanical link 140, through first arm 120, through anchor 150, and back to the electrical source.
Because the width of first arm 120 is narrower than the width of second arm 130 (with the exception of flexure portion 180), the current density in first arm 120 will be greater than the current density in the wider portion of second arm 130. The larger current density in first arm 120 causes first arm 120 to become hotter than second arm 130. For this reason first arm 120 is sometimes referred to as a xe2x80x9chotxe2x80x9d arm 120 and second arm 130 is sometimes referred to as a xe2x80x9ccoldxe2x80x9d arm 130. The higher level of heat in first arm 120 causes the thermal expansion of first arm 120 to be greater than the thermal expansion of second arm 130.
Because first arm 120 and second arm 130 are joined at the free end of thermal beam actuator 100 by mechanical link 140, the differential expansion of first arm 120 and second arm 130 causes the free end of thermal beam actuator 100 to move in an arc-like trajectory parallel to the surface of substrate 110. When the electric current is switched off, the heating of first arm 120 and second arm 130 ceases. Then first arm 120 and second arm 130 cool down. As first arm 120 and second arm 130 cool down they return to their equilibrium positions.
The essential requirement for generating deflection in thermal beam actuator 100 is to have one arm expand more than the other arm. Prior art thermal beam actuators such as thermal beam actuator 100 are capable of producing lateral deflections (i.e., deflections parallel to the plane of substrate 110) on the order of five microns (5.0 xcexcm) with typical drive voltages that are less than seven volts (7.0 v).
FIG. 2 illustrates a schematic plan view of thermal beam actuator 100. Anchor 150 is coupled to electrical connector 210 and anchor 160 is coupled to electrical connector 220. Electrical connector 210 and electrical connector 220 are coupled to a source of electric current (not shown in FIG. 2). Portions of the surface of second arm 130 adjacent to substrate 110 are formed into a plurality of support dimples 230 spaced along the length of second arm 130. The plurality of support dimples 230 position second arm 130 above substrate 110 and serve as near frictionless bearings as second arm 130 moves laterally across the surface of substrate 110. An exemplary placement of the plurality of support dimples 230 along second arm 130 is shown in FIG. 2. Although the support dimples 230 are located under second arm 130, they are shown in FIG. 2 in solid outline (rather than in dotted outline) for clarity.
FIG. 3 illustrates a cross sectional view of thermal beam actuator 100 taken along line Axe2x80x94A of FIG. 2. FIG. 3 shows how second arm 130 is positioned above substrate 110 by the plurality of support dimples 230.
Thermal beam actuator 100 may be constructed using the following typical dimensions. First arm 120 is one hundred ninety microns (190 xcexcm) long, two microns (2 xcexcm) wide, and two microns (2 xcexcm) thick. Flexure portion 180 of second arm 130 is forty microns (40 xcexcm) long, two microns (2 xcexcm) wide, and two microns (2 xcexcm) thick. The remaining portion of second arm 130 is one hundred fifty microns (150 xcexcm) long, fifteen microns (15 xcexcm) wide, and two microns (2 xcexcm) thick. The width of gap 170 determined by mechanical link 140 is two microns (2 xcexcm). Each support dimple 230 is five microns (5 xcexcm) long, five microns (5 xcexcm) wide, and one micron (1 xcexcm) thick. Anchor 150 and anchor 160 are each fifteen microns (15 xcexcm) long and fifteen microns (15 xcexcm) wide. Electrical connector 210 and electrical connector 220 are each one hundred microns (100 xcexcm) long and one hundred microns (100 xcexcm) wide. These dimensions are exemplary. Other dimensions may be used to construct thermal beam actuator 100.
As shown in FIG. 4 and in FIG. 5, thermal beam actuator 100 can be operated in two modes. In the basic xe2x80x9cthermo-elasticxe2x80x9d mode (illustrated in FIG. 4) electric current is passed through thermal beam actuator 100 from electrical connector 210 to electrical connector 220 (or vice versa). The higher current density in first arm 120 (the narrower hot arm) causes it to heat and expand more than second arm 130 (the wider cold arm). As previously explained, the differential expansion of first arm 120 and second arm 130 causes the free end of thermal beam actuator 100 to move in an arc about flexure portion 180 that is attached to anchor 160. The deflected position of thermal beam actuator 100 is shown in dotted outline 410 in FIG. 4. Switching off the electric current allows thermal beam actuator 100 to return to its equilibrium state.
The alternate xe2x80x9cthermo-plasticxe2x80x9d mode of operation (illustrated in FIG. 5) is used to create a permanent deformation in first arm 120 (the narrower hot arm) of thermal beam actuator 100. The permanent deformation is accomplished by supplying enough electric current to cause plastic deformation of the polysilicon of first arm 120. In general, the amount of electric current necessary to create a permanent deformation of first arm 120 is slightly higher than the electric current needed to generate the maximum deflection of the end of thermal beam actuator 100. When the electric current is switched off, thermal beam actuator 100 is left permanently xe2x80x9cback bentxe2x80x9d from its original position due to bowing or buckling of first arm 120. The amount of deformation or xe2x80x9cback bendingxe2x80x9d depends on the amount of over-current that is applied. The xe2x80x9cback bentxe2x80x9d position of thermal beam actuator 100 is shown in dotted outline 510 in FIG. 5. After back bending, thermal beam actuator 100 can be operated in the basic xe2x80x9cthermo-elasticxe2x80x9d mode. Back bending is particularly useful for the one time positioning of thermal beam actuator 100 and as a tool for the assembly of complex devices.
FIG. 6 illustrates how a cantilever beam 630 may be used to experimentally measure the force that can be generated at the free end of activated thermal beam actuator 100. Cantilever beam 630 is positioned parallel to second arm 130 and affixed to anchor 640 which is in turn affixed to substrate 110. Cantilever beam 630 is typically five microns (5 xcexcm) wide. One micron (1 xcexcm) square support dimples (not shown) are placed under cantilever beam 630 to support cantilever beam 630 above substrate 110 and to minimize frictional losses as cantilever beam 630 is moved across the surface of substrate 110 by thermal beam actuator 100.
As shown in FIG. 6, second arm 130 of thermal beam actuator 100 is formed having portions that define a pointed tip 610 to facilitate a measurement of the amount of deflection of the free end of thermal beam actuator 100. Similarly, the free end of cantilever beam 630 is formed into a pointed tip 650 to facilitate a measurement of the amount of deflection of cantilever beam 630. Second arm 130 of thermal beam actuator 100 is also formed having portions that define a contact extension 620 for abutting cantilever beam 630 when thermal beam actuator 100 is deflected. The physical gap between contact extension 620 and cantilever beam 630 is typically two microns (2 xcexcm).
FIG. 7 illustrates a deflection scale 710 for measuring the deflection of thermal beam actuator 100 and cantilever beam 630. Deflection scale 710 is fabricated on the surface of substrate 110. The scale marking of deflection scale 710 are typically two microns (2 xcexcm) wide. Deflection scale thermal beam indicator 720 fabricated on the surface of substrate 110 marks the equilibrium position of thermal beam actuator 100. Deflection scale cantilever beam indicator 730 fabricated on the surface of substrate 110 marks the equilibrium position of cantilever beam 630.
Activation of thermal beam actuator 100 causes second arm 130 to deflect toward cantilever beam 630. Deflection of second arm 130 causes contact extension 620 to abut cantilever beam 630 and to deflect cantilever beam 630. The deflection of thermal beam actuator 100 and the deflection of cantilever beam 630 are accurately measured by observing the position of tip 610 and tip 650 on deflection scale 710. In this manner it is possible to measure the magnitude of tip deflection versus applied electric current and power. This information enables one to obtain the amount of force xe2x80x9cFxe2x80x9d (in micro Newtons) exerted by thermal beam actuator 100 on cantilever beam 630 using the following equation:                     F        =                                            E              ⁢                              xe2x80x83                            ⁢              h                        4                    ⁢                                    (                              b                k                            )                        3                    ⁢          d                                    (        1        )            
where xe2x80x9cFxe2x80x9d is the force applied to cantilever beam 630, xe2x80x9cExe2x80x9d is the Young""s modulus of elasticity of cantilever beam 630, xe2x80x9chxe2x80x9d is the width of cantilever beam 630, xe2x80x9cbxe2x80x9d is the thickness of cantilever beam 630, xe2x80x9ckxe2x80x9d is the suspended length of cantilever beam 630, and xe2x80x9cdxe2x80x9d is the deflection of cantilever beam 630. Equation (1) ignores losses due to friction as cantilever beam 630 moves across the surface of substrate 110.
Consider a thermal beam actuator 100 having the following dimensions. First arm 120 is two hundred microns (200 xcexcm) in length, two microns (2 xcexcm) in width and two microns (2 xcexcm) in thickness. Second arm 130 is one hundred seventy microns (170 xcexcm) in length, fourteen microns (14 xcexcm) in width and two microns (2 xcexcm) in thickness. Flexure portion 180 is thirty microns (30 xcexcm) in length, two microns (2 xcexcm) in width and two microns (2 xcexcm) in thickness. A typical applied voltage of four and three tenths volts (4.3 v) produces an applied current of three and eight tenths milliamps (3.8 mA) and an applied power of sixteen and three tenths milliwatts (16.3). This causes the tip of thermal beam actuator 100 to be deflected by eight microns (8 xcexcm).
When thermal beam actuator 100 deflects cantilever beam 630 by eight microns (8 xcexcm), the value of xe2x80x9cdxe2x80x9d in Equation (1) is eight microns (8 xcexcm). Equation (1) may then be used to calculate that a deflection of eight microns (8 xcexcm) corresponds to a force of four micro Newtons (4 xcexcN) exerted by thermal beam actuator 100.
An array of thermal beam actuators may be used in applications that require more force than a single thermal beam actuator can supply or when linear motion is required. FIG. 8 provides an example of how a plurality of prior art thermal beam actuators (810, 820, 830, 840, 850) may be grouped together to form a thermal beam actuator array 800. The free end of each thermal beam actuator in array 800 is formed having portions that define a connecting link (860a, 860b, 860c, 860d, 860e) that is coupled to a common mechanical yoke 870. The combined force exerted by the thermal beam actuators in array 800 is exerted on mechanical yoke 870. Mechanical yoke 870 is a critical component in thermal beam actuator array 800 because it combines the motion and the force of the thermal beam actuators in array 800 in a linear deflection. Each thermal beam actuator in thermal beam actuator array 800 comprises a thermal beam actuator 100.
Some micro-electro-mechanical applications may require the use of a single thermal beam actuator instead of an array of thermal beam actuators. There is therefore a need in the art for improvement in the design and construction of thermal beam actuators. In particular, there is a need in the art for a thermal beam actuator that can generate more force than presently existing thermal beam actuators can generate. There is also a need in the art for a thermal beam actuator that can optimize power consumption, tip deflection and generated force.
The present invention comprises a system and method for providing an improved thermal beam actuator that is capable of providing force to move elements of a micro-electro-mechanical device. The thermal beam actuator of the present invention is capable of generating a force that is greater than the force that can be generated by prior art thermal beam actuators.
In prior art electrothermal actuators, electric current flows through the hot arm and through the cold arm. Differential thermal expansion of the hot arm and the cold arm deflects the end of the cold arm. The present invention provides an additional hot arm to provide a return path for the electric current so that the cold arm does not conduct electric current. This eliminates the parasitic electrical resistance of the cold arm. Because the cold arm of the present invention does not conduct electric current a more narrow flexure portion of the cold arm may be used. This increases the mechanical efficiency of the thermal beam actuator. The present invention also optimizes power consumption, tip deflection and generated force of the thermal beam actuator.
Accordingly it is an object of the invention to provide a thermal beam actuator that comprises at least two hot arms and a cold arm.
A further object is to provide a thermal beam actuator in which a cold arm of the thermal beam actuator does not conduct electric current.
An additional object of the invention is to provide a thermal beam actuator in which the flexure portion of the cold arm of the thermal beam actuator has a width that is less than the width of a flexure portion of a prior art thermal beam actuator.
An additional object of the invention is to provide a thermal beam actuator in which the flexure portion of the cold arm of the thermal beam actuator has a width that is greater than the width of a flexure portion of a prior art thermal beam actuator.
Another object of the invention is to provide a thermal beam actuator array comprising a plurality of thermal beam actuators in which each thermal beam actuator has at least two hot arms and a cold arm.
A further object of the invention is to provide a bidirectional thermal beam actuator which may be selectively deflected in either of two directions.
Another object of the invention is to provide a bidirectional thermal beam actuator array comprising a plurality of bidirectional thermal beam actuators.
A further object of the invention is to provide a method for operating a thermal beam actuator in which no electric current is conducted through the cold arm of the thermal beam actuator.
Further objects of the invention will become apparent from the description of the invention which follows.